Hand held weather instruments exist that measure, among other variables, the relative humidity of ambient air. A humidity sensor is used to measure the relative humidity of the ambient air. While a humidity sensor is not used to measure temperature, its measurement of the relative humidity is strongly effected by the temperature of the humidity sensor itself. For example, a common type of humidity sensor determines relative humidity by determining the amount of water absorbed into a dielectric material of a capacitor. Since the ability of the dielectric material to absorb water is a function of the temperature of the dielectric material, the measurement of relative humidity is dependent upon the temperature of the humidity sensor. At higher temperatures less water can be absorbed, and at lower temperatures more water can be absorbed. Therefore, if the humidity sensor were warmer than the ambient air being measured, less water absorbs into the dielectric material and the humidity sensor responds as though the relative humidity of the ambient air were in fact lower than it actually is, resulting in an inaccurate relative humidity reading.
Inaccuracies caused by temperature differentials between the ambient air and the humidity sensor itself are especially prevalent in hand held weather instruments. Hand held weather instruments are often stored in environments, such as a user's pocket or a house, where the temperature will be much different from the temperature of the ambient air. When stored in such an environment, the humidity sensor will approach, or obtain, a thermal equilibrium with the storage environment. If the weather instrument is then removed from the storage environment and placed in the ambient air, the accuracy of the relative humidity reading of the ambient air will be compromised because the humidity sensor will not be at the same temperature as the ambient air. This is known as thermal lag. Thus, in order to obtain an accurate relative humidity reading of the ambient air with such a weather meter, one must wait until the humidity sensor reaches thermal equilibrium with the environment. However, waiting for a humidity sensor to reach thermal equilibrium with the ambient air can take a significant amount of time. Moreover, determining when thermal equilibrium is achieved can be difficult to recognize.
While methods and apparatus have been developed to compensate for thermal lag, existing methods and apparatus are unsatisfactory and/or are less than optimal. In one existing weather instrument, the problem of thermal lag is minimized by locating the humidity sensor exterior to the housing of the weather instrument so that the humidity sensor is in direct contact with the ambient air. This allows the humidity sensor to more quickly obtain thermal equilibrium with the ambient air. An example of such a weather instrument is disclosed in U.S. Pat. No. 6,257,074, which is hereby incorporated herein by reference in its entirety. However, locating the humidity sensor exterior to the housing of the weather instrument introduces a number of problems, such as exposing the humidity sensor to damage from physical contact, static discharge, and contact with contaminants, including liquid water and/or salt water. These problems are exasperated by the fact that humidity sensors are fragile and often expensive.
Thus a need exists for a method and apparatus for measuring relative humidity of a mixture that solves these and other problems.